paleontology 6: The rise of the reptiles (v1.0)

Reptiles are egg laying, cold blooded, typically scale covered, vertebrate tetrapod and constitute a large class of animals. Examples range from dinosaurs to turtles, tortoises, lizards, alligators, crocodiles, snakes, iguana, and chameleon. They are an extraordinary evolutionary success. The emergence of reptiles is a major evolutionary change. The study of reptiles and amphibians is called Herpetology. How and when did reptiles first evolve? 

As indicated in a previous essay, a key evolution transition was when lobed fin fish evolved into tetrapod and allowed vertebrates to occupy new ecological niches on land in the Devonian and Carboniferous periods between 400 and 300 million years ago. The joggin fossils found in the 19^th century in Nova Scotia from the Carboniferous period were a key one. Fossils of nearly 200 species were identified here in the fossil tree remains of a carboniferous forest and one of the earliest known reptiles were found here from about 312 million years ago.

A key difference between amphibians and reptiles is amphibians have dual existence on land and water with external fertilization in water while reptiles have existence mostly on land with internal fertilization in shelled eggs on land. Reptiles also don’t go through a larval stage or have metamorphosis. Reptiles first arose from an earlier amphibian tetrapod in the swamps of the carboniferous forests in the Pennsylvanian subperiod of the carboniferous period. Increasing evolutionary pressure and the vast untouched niches of the land powered the evolutionary changes in amphibians to gradually become increasingly land based. 

Amniotes are a large group of tetrapod vertebrates that comprise the majority of living terrestrial vertebrates and include reptiles, birds, and mammals. They are separate from Lissamphibians, examples of which are amphibians like frogs, toads, salamanders, and newts.

In the early Triassic period (which stretches from the end of the Permian period 251 million years ago to the start of the Jurassic period 201 million years ago) early dinosaurs appeared. Dinosaurs fascinate contemporary people! Yes, some dinosaurs were excessively big. But many were more human sized. The emergence of dinosaurs is a major evolutionary change. Dinosaurs are defined as a member of a clade that contains the most recent common ancestor of modern birds and Triceratops as well as all their descendants. Dinosaurs were an evolution that was able to walk upright on two legs. There is a selective advantage to using two legs. It is better adapted to running. It also frees up the forearms to grasp. It also helps with breathing.

The first dinosaurs were not large and emerged 230 million years ago. They descended from a clade called dinosauromorphs (with some of the best fossils in Argentina) which belong to archosaurs (which evolved about 250 million years ago. They include crocodiles and birds today). Dinosauromorphs appeared in the Middle Triassic. The dinosaurs stayed around until the late Cretaceous period about 66 million years ago. That means they were living and evolving for 165 million years! Over 1000 species have been found and they were a big evolutionary success. But they did not fully go extinct through a mass extinction about 66 million years ago, as many people think. Birds evolved from small dinosaurs and are the living “dinosaurs” of today. More on this in the next essay. The mass extinction event 66 million years ago I talked about was due to an asteroid impacting the earth (at the Chicxulub crater in Yucatan peninsula in Mexico – asteroid was 6 miles in diameter) that caused massive dust plumes, debris and gas in the atmosphere that altered the climate and disrupted the food chain. 

Marine reptiles are an interesting class of reptiles. These are reptiles that adapted themselves to a marine environment. Mary Anning (Mary is the person in the tongue twister “She sells seashells by the seashore”) in the 19^th century played a huge role in finding many marine reptile fossils in England from the Jurassic period (from 200 to 145 million years ago) and selling them to scientists. Examples of fossils found by her include the Ichthyosaur (oldest known fossil is 245 million years ago) and Plesiosaur (oldest known fossil is 200 million years ago). Another example of an extinct early marine reptile is Mesosaurus (early Permian – 299 to 270 million years ago). Mosasaurs came around 110 million years ago (Komodo dragons today are related). Sea Turtles are also marine reptiles and emerged about 105 million years ago. Sea Snakes emerged about 95 million years ago.

Flying reptiles are an interesting class of reptiles. These are reptiles that adapted themselves to flight. The first reptiles showing powered flight were pterosaurs (emerged 210 million years ago in late Triassic period). The speculation is that flight developed from the ground up likely along extensive sea cliffs. They range from small animals to exceptionally large ones (wingspan of 36 feet!!). They nested on land and laid eggs.

In this essay I covered the rise of reptiles including marine and flying ones, and also dinosaurs. It spans a period from 312 million years in the Carboniferous period to 66 million years when an asteroid hit earth which was the end of the Cretaceous period.